This invention relates in general to sealing systems for volatile liquid storage tanks, and in particular, is directed to a sealing system adapted to provide a vapor seal about the periphery of the roof of a floating roof type of liquid storage tank.
It is the common practice to store volatile liquids such as gasoline, jet fuels, light crude oil and the like in large, above ground, welded or riveted, steel storage tanks. Such tanks are generally characterized by cylindrical, welded or riveted steel sidewalls and a floating steel roof which is conventionally provided with suitable buoyancy devices to allow the floating type of roof to rise and fall within the tank sidewalls as the quantity of stored product increases and decreases.
Inherent in the floating roof type of tank design is a narrow annular space which is defined between the periphery of the floating roof and the tank sidewalls. This annular space must be great enough to accommodate any protrusions which extend inwardly of the tank sidewalls, for example, a rivet head as well as to provide suitable clearance to adjust for any irregularities from true round configuration in either the tank sidewalls or in the floating roof. In order to prevent loss of vapors through the annular space to thereby contaminate or pollute the ambient air, it has been the usual practice to provide some type of vapor barrier between the outer periphery of the floating roof and the inner periphery of the tank sidewalls. Many different sealing constructions have been developed by prior workers in the art to provide a suitable vapor barrier and these annular seals have generally been designated in the trade as the primary seal.
Recently, as concerns for environmental conditions have increased, more stringent requirements for hydrocarbon emission control to prevent vapor loss in and about the primary seal have been enacted. One such standard that is commonly followed at the present time has been promulgated by the California Air Resources Board and is generally designated as "Rule 463." In C.A.R.B. Rule 463, specific design parameters for floating roof tanks have been enumerated and tank secondary seal requirements have been set forth.
The C.A.R.B. Rule 463 requirements have now become so popular and so well recognized as to substantially be incorporated into all floating roof tank design standards in those communities wherein floating roof tank regulations have been adopted and enforced. The secondary seal design disclosed in U.S. Pat. No. 4,308,968 is exemplary of the type of secondary seal construction that must now be provided.
Most of the prior art secondary seal constructions, including the construction disclosed in U.S. Pat. No. 4,308,968, include an annular support structure comprising a plurality of circularly adjacent, flexible, steel, support plates. The support plates extend upwardly from the periphery of the floating roof structure for a distance that is usually between eighteen to twenty-four inches in order to assure adequate flexibility. The support plates are each bottomly bent to cause the plate to continuously flex radially outwardly toward the tank sidewalls. The support structure terminates upwardly in one or more flexible wipers, which wipers are maintained in intimate contact with the inner surface of the tank sidewalls by the flexure of the support plates to thereby provide a continuous, vertically sliding, vapor barrier to prevent the escape of volatile vapors at the secondary seal.
Of course, it will be appreciated that the secondary seal wipers must contact the inner surface of the tank sidewalls as the floating roof rises and falls to remain effective. Accordingly, the storage capacity of the tank itself is thereby limited by the elevated position of the wipers above the top of the floating roof. Care must be exercised to prevent the entrance of a quantity of product into the tank that is sufficient to elevate the floating roof to raise the secondary seal wipers above the top of the tank sidewall construction. It will be noted that, any vertical space that must be utilized to accommmodate the height that the secondary seal wipers are elevated above the top of the floating roof tank results in a corresponding loss of storage capacity within the tank. Thus, the installation of the secondary seals which employ the vertically elevated, flexible wipers has caused a corresponding decrease in storage capacity of the tank itself.